1. Field of the Invention
The present invention relates to a drill bit having step-shaped cutting edges that are axially provided.
2. Description of Related Art
A drill is a known tool used as a perforation tool. A two-edge drill bit is frequently used (see Patent Document JP2008-36759A, for example).
A typical conventional drill bit (“drill bit” is also designated as “drill” hereinafter) has two cutting blades having a specified point angle at a tip of the drill bit shown as a drill bit B in FIGS. 10 and 11.
On the other hand, it is required in various industry fields to perforate a work piece W to be perforated, in which a metal material W1 and a fiber reinforced resin composite material W2 are laminated as shown in FIGS. 10 and 11, by one drill all at once. That is because a fiber reinforced resin composite material for reduction of weight and a metal (plate) material disposed on an inner surface or an outer surface of the composite material are to be perforated and a connection member such as a bolt, etc. is to be inserted so as to structure the composite structure by combining these materials. Therefore, it is required to perforate each material at a corresponding position precisely and efficiently. Aluminum is an example as the metal material W1 and a carbon fiber reinforced resin composite material is an example of the fiber reinforced resin composite material W2.
Various kinds of materials can be included as a work piece W. For example, the metal material W1 is formed of a plurality of layers, the fiber reinforced resin composite material W2 is formed of a plurality of layers, or different material layers are piled up in each case. And a drill that can be applicable in any case is desirable.
It is desirable to hold the work piece W using holding tools CL at a position apart from the perforating position, as shown in FIGS. 10 and 11, for holding the work piece W during a perforation work. It is not desirable to put a support material on a side that the drill will penetrate. That is because it causes demerits such as material wasting, additional perforation work and additional drill cuttings.
In the case a work piece to be perforated is composed of a metal material W1 disposed on one side and a fiber reinforced resin composite material W2 disposed on the other side, as shown in FIGS. 10 and 11, it is possible to select a perforation direction from a metal material W1 side or a resin composite material W2 side. However, at a manufacturing site of structures using such a composite member, at an aircraft factory for example, it is not necessarily possible to select the perforation direction from a viewpoint of a structure itself or a positional relation to other surrounding members. Thus a drill that is able to perforate a work piece excellently from either direction is requested.
As shown in FIGS. 10 and 11, when perforating the work piece W from the side of the fiber reinforced resin composite material W2 using a typical conventional drill bit B having two cutting blades having a specified point angle at a tip of the drill, following phenomena can be seen.
At first, as shown in FIG. 10A, the drill B perforates the fiber reinforced resin composite material W2 and the drill is advanced to the tip direction.
When the tip of the drill B reaches to the metal material W1, the drill B makes the metal material W1 bend and swell toward the tip direction while the perforation of the metal material W1 proceeds, as shown FIG. 10B and then FIG. 10C. At this point, since drill cuttings tend to become linked and long, there is a concern whether or not the drill cuttings of metal can be ejected (cleared) smoothly toward the rear of the drill B through the perforated hole in the fiber reinforced resin composite material W2. In the case where the drill cuttings are not ejected smoothly, cutting efficiency will be decreased. In addition, it will be a problem if drill cuttings be remained between the metal material W1 and the fiber reinforced resin composite material W2.
After that, the tip of the drill B reaches to the surface of the metal material W1 and makes a small hole thereon. Then the metal material W1 purposes to return to the original position, as shown in FIG. 11A, by virtue of a tension generated in the metal material W1 caused by the bending deformation. There is a concern at this time that the drill is stopped or the cutting blade is damaged by the sharply increased load to the drill. And also there is a problem that such a hole perforated and returned from bending cannot be finished with high precision. The drill B does not have a finishing edge to finish the hole on the metal material W1 at the time shown in FIG. 11A. Thus it becomes necessary to finish the hole after perforation by the drill B. After that, as shown in FIG. 11B, the maximum diameter portion of the cutting blade of the drill B cuts through the metal material W1 to complete the perforation process.
There is a concern, when using the typical conventional drill B to perforate the work piece W from either the metal material W1 side or the composite material W2 side, that delamination of the composite material W2 may occur and it is difficult to finish the hole precisely. Thus there is a limitation of high precision perforation of the metal material W1 or the composite material W2.
The degree of bending of the metal material W1 when it bends toward the tip direction of the drill depends on characteristics of the drill as well as characteristics of the metal material W1.
Table 1 shows a degree of bending (mm) of the metal material W1 under the following conditions.
Table 1 shows a maximum degree of bending of the metal material W1 under combined conditions selected from materials of an aluminum (A7075) and titanium alloy (6-4Ti), thickness of the metal material W1 of 3 (mm) and 4 (mm), and the diameter of the drill B of 4 (mm) and 5 (mm). The metal material W1 bends in accordance with the movement of the tip of the drill B as shown by FIGS. 10B and 10C; however, the degree of bending shown in Table 1 means the maximum displacement of the metal material W1 in the drill axis direction. A distance S between the holding tools CL in FIG. 10A was 100 (mm) and an advancing speed of the drill was set at 0.15 (mm/rev).
As can be seen by Table 1, the degree of bending of the metal material W1 at perforation by the drill in the case using titanium alloy (6-4Ti) becomes larger than the case using aluminum (A7075). And the degree of bending becomes large as the thickness of the metal material W1 becomes thin. The results are caused by the bending rigidity of the metal material W1.
On the other hand, the degree of bending of the metal material W1 at perforation becomes large as the drill diameter becomes large. That is because a force to bend the metal material W1, that is a thrust force, becomes large as the drill diameter becomes large.
TABLE 1Degree of bending (mm)φ4 twistφ5 twistW1 thicknessA70750.5610.6933 mm6-4Ti0.5860.725W1 thicknessA70750.2360.2924 mm6-4Ti0.2740.3